Check if QML Item is being drawn

0
votes

I have a set of QML items distributed all over my UI. They display data from a remote device and their content needs to be updated regularly. The Items are spread on several tabs and hidden in nested ListView instances, so most of them won't be visible to the user all the time.

In order to keep the bandwidth low I want to update only those items that are currently visible to the user.

I am looking for the right hook to get the information which of these Items is currently displayed from within the Item, without relying on information from the parents. If they were all placed in ListView delegates I could use the delegate's Components onCompleted and onDestroyed signals. Since this is not the case I am stuck at finding out how to get this information.

Am I missing something here? Is there an onPaintFinished signal or something similar? My workaround would be to add that logic to the parent containers, but that would be tedious, since there are several kinds of container that can contains these display Items.

1
qtqml
if part of your items are placed in tabs so in all probability they do not yet exist. - folibis
They do, since the tabs are implemented as a StackLayout. - Herr von Wurst
Ok, so you can update only the items of the current tab. Anyway, it would be better if you provide Minimal, Reproducible Example - folibis

1 Answers

1
votes

Instances that are on delegates of a ListView will not exist until they would be in the visible range or the cache range around the visual area of the list view. If the delegate moves outside of that range, it is destroyed. So, no need to worry about instances hidden there.

Furthermore, items are currently not visible are also not drawn. They are not entered into the scene graph, and hence, not rendered. So, instances of your items appearing on tabs that are currently not current will also not be drawn. However, these items do still exist of course.

Figuring out if an item is effectively visible or not is quite a hard problem though. QML delegates part of that to OpenGL (clipping for instance). There is not feedback on the result of that. You could in theory lift that information out of the renderer, but that would require customizing that and that is very hard. You could take a look at the heuristics that GammaRay uses to warn about items not being visible. Perhaps you can take some inspiration from that.